The big idea: Within any species, individuals are not all identical — they show variation.
Natural selection can only work if there are differences for it to choose between. So variation is the raw material that evolution acts on.
But only heritable variation matters — differences that can be passed to offspring through genes. This comes from three things: mutation, meiosis and sexual reproduction (random fertilisation).
- Variation
- The differences that exist between individuals of the same species.
- Heritable variation
- Variation caused by differences in genes (alleles), so it can be passed to offspring. Only this kind matters for evolution.
- Allele
- A different version of the same gene — e.g. an allele for a dark coat versus an allele for a light coat.
- Mutation
- A random change in the DNA base sequence. It is the only source of genuinely new alleles.
Heritable vs not heritable: A difference is only raw material for evolution if it can be inherited.
Heritable: caused by alleles in the gametes (sex cells) — e.g. eye colour, blood group. These pass to the next generation.
Not heritable: caused only by the environment during your lifetime — e.g. a suntan, a scar, muscles built by exercise. These are not passed on, so natural selection cannot act on them.
There are three sources of heritable variation, and the key exam distinction is between making something NEW and just SHUFFLING what already exists.
Only mutation creates brand-new alleles. Meiosis and sexual reproduction do not make new alleles — they rearrange existing ones into new combinations.
The three sources, in order
- Mutation — a random change in the DNA base sequence makes a brand-new allele. This is the only source of completely new genetic variation.
- Meiosis — during the formation of gametes, crossing over swaps segments between homologous chromosomes, and independent assortment sorts the chromosome pairs at random. Both shuffle existing alleles into new combinations.
- Random fertilisation — any sperm can fuse with any egg, so two random sets of alleles combine. This shuffles existing alleles yet again.
| Source of variation | What it does | New allele or just a new mix? |
|---|---|---|
| Mutation | A random change in the DNA base sequence creates a brand-new allele of a gene | Makes a genuinely NEW allele — the only source that does |
| Meiosis: crossing over | Homologous chromosomes swap matching segments, mixing alleles along a chromosome | Shuffles EXISTING alleles into new combinations |
| Meiosis: independent assortment | Each chromosome pair lines up and separates at random, so gametes get random mixes of maternal and paternal chromosomes | Shuffles EXISTING alleles into new combinations |
| Random fertilisation | Any sperm can fertilise any egg, combining two random sets of alleles | Shuffles EXISTING alleles into new combinations |
New vs shuffled — the line the exam tests: Mutation = NEW. It is the only way a population can gain an allele it never had before. Mutation is the ultimate source of all genetic variation.
Meiosis + sexual reproduction = SHUFFLED. They take alleles that already exist and deal them into new combinations, so siblings differ from each other and from their parents — but no new allele is created.
If a question asks how sexual reproduction increases variation, the answer is recombination / new combinations of existing alleles — not new alleles.
Why only mutation makes the raw NEW material: Think of a deck of cards.
Meiosis and fertilisation just re-deal the same deck — endless new hands, but the same 52 cards.
Mutation is the only thing that can add a brand-new card to the deck.
Evolution needs new cards over the long run, so mutation is the ultimate source of all heritable variation.
| Type of variation | Pattern | Typical cause | Example |
|---|---|---|---|
| Continuous | A smooth range of values between two extremes | Many genes acting together, often with an environmental effect | Human height, leaf length, body mass |
| Discontinuous | A few distinct categories with no in-betweens | Usually one or a few genes, little environmental effect | Human blood group (A/B/AB/O), ability to roll the tongue |
Only germ-line mutations are heritable: A mutation only counts as a source of variation for evolution if it happens in a gamete or a germ-line (gamete-forming) cell, because only then is it passed to offspring.
A mutation in an ordinary body (somatic) cell — for example a skin cell — affects only that individual and is not inherited, so natural selection across generations cannot act on it.
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How this is tested: On Paper 1A (multiple choice) a 1-mark item asks you to identify how mutation and sexual reproduction generate variation — mutation makes new alleles; sexual reproduction (meiosis + fertilisation) makes new combinations of existing alleles.
On Paper 2 mutation appears in a longer Explain answer about evolution. Score the separate points: mutation is a random DNA change; it is the only source of new alleles; only heritable (germ-line) mutations count; most are neutral or harmful but a few are beneficial; mutation supplies the raw variation natural selection acts on, so without mutation a population cannot evolve.
IB-style question — explain the role of mutation in evolution
Explain the role of mutation in providing the variation needed for evolution. [5]
How to score the marks
- What a mutation is. A mutation is a random change in the DNA base sequence, which can create a new allele of a gene.
- Why it is special. Mutation is the only source of completely new alleles — meiosis and sexual reproduction only shuffle existing alleles, they cannot create new ones.
- Which mutations count. Only mutations in gametes / germ-line cells are heritable and passed to offspring; mutations in body (somatic) cells are not inherited.
- Their effect varies. Most mutations are neutral or harmful, but occasionally one is beneficial in a given environment.
- Link to evolution. Mutation supplies the raw heritable variation that natural selection acts on; a beneficial allele can be selected for and increase in frequency. Without mutation, no new variation arises and the population cannot evolve. (Award 1 mark per distinct point, up to 5.)
Final answer
A mutation is a random change in the DNA base sequence that can make a new allele; it is the only source of new alleles (meiosis/sex just shuffle existing ones); only heritable (germ-line) mutations are passed on; most are neutral or harmful but some are beneficial; mutation supplies the raw variation natural selection acts on, so without it a population cannot evolve.
✓ Why this scores full marks: Each sentence is a separate, distinct point — what a mutation is, why it is the only new source, which mutations are heritable, that their effects vary, and the link to natural selection.
A 5-mark Explain needs five scoring points with reasons, not one idea ('mutation causes variation') written five ways.